Sepsis (systemic inflammatory response to infection) has an incidence of 750,000 cases and is the leading cause of death in critically ill patients. Treatment of sepsis has been limited and remains largely supportive in nature. Improved understanding of the mechanisms underlying inflammation in sepsis and preclinical investigation of interventions designed to reduce mortality are part of the NHLBI mission. Preliminary studies show that the costimulatory molecules, CD80 and CD86 are important in the innate immune response to sepsis. CD80/86-/- mice have improved survival, reduced inflammatory cytokine production and less NF-kappaB activation after polymicrobial sepsis produced by cecal ligation and puncture (CLP). An in vitro model using neutrophil (PMN)/macrophage co-culture leads to macrophage activation by a CD80/86 dependent pathway. During clinical investigation of sepsis we observed that PMN from septic humans have increased expression of a CD28 (a CD80/86 ligand) and mortality correlated with soluble CD28 levels. We hypothesize that macrophage expressed CD80/86 are involved in the innate immune response to sepsis. To determine the specific importance of CD80 and CD86 we will use mice congenitally deficient in these molecules as well as siRNA and inhibitory antibodies to modulate CD80 and CD86 expression in macrophages. We will investigate the expression of the CD80/86 system in human sepsis by flow cytometry and confocal microscopy and compare regulation in humans and mouse to validate the CLP model. We will then assay the effect of PMNs from normal and septic human subjects on macrophages in vitro and assess the role CD80 and CD86 using using siRNA and blocking antibodies in co-culture experiments. This is a proposal for investigation in sepsis and training which includes a completion of a Masters of Science in Clinical Investigation.The course work and the experience in the laboratory under the mentorship of Dr. Weiden are essential for developing my abilities in identifying pathophysiologic mechanisms in model systems. This grant will foster a career focused on understanding the mechanisms underlying inflammation in sepsis with the goal of developing intervensions that reduce mortality in this important disease. [unreadable] [unreadable] [unreadable]